The hydrogen fuel cell or polymer electrolyte membrane fuel cell (“PEMFC” or “PEM” fuel cell) is one of the most promising future alternative energy sources, particularly attractive for automobile applications due to its high efficiency, high energy density, and low or zero emissions. However, its relatively low power output has prevented it from many practical applications. Typical applications for PEM fuel cells are backup power, portable power, distributed generation and transportation. The advantages of PEM fuel cells are that they can be used with solid electrolyte, have high power density, a low operation temperature, quick start-up and almost pollution-free emissions. The disadvantages are expensive catalysts, sensitive to impurities and they have low temperature waste heat. The hydrogen gas for the fuel cells can be obtained from natural gas reforming, water electrolysis and photo-catalytic water splitting.
In fuel cells, power is generated via the conduction of protons—positively charged hydrogen ions (H+)—through a polyelectrolyte membrane, commonly composed of a sulfonated tetrafluoroethylene based fluoropolymer-copolymer (NAFION®). The function of the fuel cell constitutes a balance between hydrogen oxidation and oxygen reduction reactions where platinum (Pt) nanoparticles are used to catalyze the reactions at the electrodes. Under ambient conditions where carbon dioxide (CO2) is present, carbon monoxide (CO) is one of the byproducts of the fuel cell operation, which is produced on the NAFION® membrane as a result of the Pt catalyzed H2 reduction at the anode, or via the reverse water gas shift reaction (RWGS) at the cathode. In either case, the rate constant for oxidation of CO is lower than that for the other reactions, especially below 400K, resulting in accumulation and subsequent migration to the electrodes where amounts, as low as 75 ppm, poison the Pt catalyst and can reduce the power output by more than 50%.
The decreased efficiency of energy conversion of a hydrogen fuel cell is mainly caused by the slow kinetics of the oxygen reduction reaction (ORR) and the presence of contaminants in the fuel stream. The presence of trace amounts of carbon monoxide in hydrogen fuel produced from the reforming process, and via routine operation in CO2 containing ambient atmosphere, can reduce fuel cell efficiency considerably when platinum is the electro-catalyst material. Recently, considerable advances have been made in fuel cell electrocatalysis moving away from conventional Pt catalysts to improved electrocatalysts (mostly nanosized), which have increased the understanding of the reaction kinetics. However, the lack of high efficiency due to contaminants is still one of the major challenges of electrocatalysis, in addition to the problem of the stability of Pt.
It is known that gold nanoparticles (Au NPs) are very effective catalysts of the CO oxidation reaction. An essential condition of this process though is the formation of hemispherical shaped particles in direct contact with metal oxide supports, where a two-step catalytic process is postulated at the perimeter of the particles which reduces the energy barrier and increases the reaction rate. Hence, despite their potential utility, these particles have not been effective in moderating the fuel cell operation since their deposition requires very high temperatures that cannot be achieved on a polymer membrane.
Au nanoparticles can be effective catalysts of the CO oxidation reaction at low temperatures when good contact with certain metal oxide (e.g., titanium dioxide, TiO2) substrates is established. It is believed that the Au nanoparticles are oblate-shaped and form a stepped interface at the contact line with the TiO2 substrate. The atomic steps then provided perimeter sites for adsorption of the reactants which enabled a two-step oxidation process to occur. In this model, the substrate first interacted with the reactants, allowing bond stretching on the support surface. This decreased the barrier to the catalytic process which occurred on active sites at the perimeter of the Au nanoparticles thereby greatly increasing the efficiency and reducing the operating temperature of the CO oxidation reaction. The model was not unique to TiO2 and a similar mechanism was recently proposed for cerium dioxide (CeO2). This type of reaction has not been used to eliminate CO poisoning of PEM fuel cells because the high temperatures involved in formation and deposition of the particles are not practical for implementation on polymer membranes and, hence, incorporation into PEM fuel cells was never attempted.
A PEMFC is a device that can directly convert the chemical energy in hydrogen to electrical energy at low temperature in the presence of catalysts. A polymer membrane, usually a NAFION® membrane, is utilized to separate ions and electrons, while two electrodes that contain loadings of carbon black and platinum (Pt) catalysts are placed at both sides of the membrane. Nanoparticles are common catalytic components of fuel cells. Most catalysts are loaded on the electrodes to catalyze the reaction. Platinum is the most common and efficient catalyst for PEM fuel cell, but Pt is precious and expensive. Generally, the two desired reactions that occur in hydrogen fuel cells are the hydrogen oxidation reaction (HOR) at the anode and the ORR at the cathode. The HOR occurs readily on Pt-based catalysts (rate constant ˜10−5 molsec−1 cm−2) and, in a fuel cell, is usually controlled by mass transfer limitations. The ORR actually can proceed by two pathways in aqueous electrolytes, which is called “four-electron pathway,” and the other one is known as the peroxide or “two-electron” pathway (O2+2H++2e−−*H2O2).
A polymer membrane, which is usually made out of NAFION®, is used to separate ions and electrons in hydrogen fuel cell. NAFION® is a sulfonated tetrafluoroethylene based fluoropolymer-copolymer. A tetrafluoroethylene (TEFLON®) backbone gives NAFION® its mechanical strength while the pendent sulfonate groups facilitate ion conduction. NAFION® does not conduct electrons, but does allow positive ions, typically protons in PEM cells, to pass through via interaction with clusters of sulfonate-ended perfluoroalkyl ether groups. The structure of these clusters is still under investigation and has been proposed to result from assemblies ranging from inverted micelles to cylindrical nanochannels. In all of the proposed models, channels with —SO−3 groups facilitate transport of positively charged species.
Operating a fuel cell with pure hydrogen exhibits the best power output; however, pure hydrogen is very expensive and difficult to store. Alternatives to pure hydrogen usually come from reformed hydrogen gas from natural gas, propane, or alcohols. Even though the reformed gas is purified, some contaminants, such as CO and CO2 species can persist in the gas feed. CO can poison the catalyst by blocking active sites on the catalyst's surface. Consequently, sites are no longer available for hydrogen adsorption and subsequent oxidation. It is known that when CO content is larger than 25 ppm, it has severe effects on Pt catalysts. CO poisoning can also occur when air is blown in the cathode, where CO2 in the air can be reduced to CO and, therefore, block the active Pt sites on the cathode.
Accordingly, there is a need for PEMFC that can operate using reformed gas containing contaminants without an intermediate purification step to remove the contaminants. There is also a need for a PEMFC that have high output power and are cost efficient to operate.